1. Field of the Invention
This invention concerns a wavelength division multiplex transmission system, and in particular routing control and defect processing.
2. Description of Related Art
Current trends in technology for increasing the capacity of optical communication systems can be broadly divided into time-division multiplex (TDM) methods, and wavelength-division multiplex (WDM) methods.
In WDM methods, signals are modulated at, for example, 10 Gbps per channel. In each channel, modulated signals at different wavelengths are superposed by a four-channel WDM optical coupler (wavelength division multiplexer), and by transmitting the resulting signal over a single optical fiber, transmission at 40 Gbps can be achieved. This WDM method has been described as being superior to the TDM method with respect to such areas as ease of upgrading, power division costs, security, and service flexibility.
In conventional systems using the WDM method, all the wavelength components of wavelength-multiplexed signals (WDM signals) have been transmitted on the same route up until the receiving end. However, in recent years there have also been numerous mesh-shape and ring-shape networks put into use which are provided with add-drop circuits (ADM, Add/Drop MUX) and cross-connect circuits (XC: cross-connect) in the WDM transmission path. In this case, an unlimited number of routes for selection, from transmission end to receiving end, can be supposed. However, all routes are different, due to the states of each route, including the number of intervening repeaters, the transmission distance, and traffic conditions on the route.
On the other hand, methods for selection of routes for transmission signals can be broadly divided into two types. In cases of end-to-end signaling-based control, for example, routes are allocated according to transmission distances (transmission path distances). In cases of IP-based routing control, for example, routes are allocated according to the number of hops until the receiving end, regardless of the transmission distance. For example, as shown in FIG. 1, when, as routes between the repeater node N1 and the repeater node N2, there is a route RT1 with a large number of hops and short transmission distance and a route RT2 with a small number of hops and long transmission distance, if selecting the route according to transmission distance the route RT1 is selected, whereas if selecting the route according to number of hops the route RT2 is selected. In either case, the same route, RT1 or RT2, is selected for all wavelength components.
However, route selection methods in conventional WDM systems have been route selection methods the selection criteria of which do not reflect important factors influencing transmission quality, such as the number of repeaters or traffic conditions.
For example, in the case of a method for route selection according to transmission distance (transmission path distance), routes are allocated solely on the basis of transmission distance, unrelated to the number of repeating switches or the traffic conditions within the transmission band. If a transmission distance is short, that route (transmission path) will be selected, no matter how many network elements (NEs) exist in the path, or how bad the characteristics are. The greater the increased number of optical amplifiers, optical switches, optical filters and other NEs existing in a network, even if as unit components they have only very small wavelength dependences, these wavelength dependences will accumulate, and the result for the entire route cannot be neglected.
In the case of methods for route selection according to number of hops also, the optical S/N ratio, used band capacity and other factors are not considered. Hence in this case also, the selected route is not necessarily optimal with respect to the transmission characteristics of optical signals. A route is selected on the basis of cost considerations alone.
Further, whichever route selection method is used, if the same route is selected for a number of data sets, a large amount of data is concentrated in a given route to result in congestion, and problems such as circuit breaks may occur. In addition, if a malfunction occurs in an NE on the transmission route, another route is used in place of that transmission route, and consequently the tendency toward congestion in the WDM transmission network is intensified, giving rise to the problem that the probability of circuit breaks is increased.
As explained above, conventional route selection methods are not suitable from the standpoint of securing transmission quality.
In particular, when a defect occurs and routes are reallocated, in extreme cases, large amounts of empty band capacity may occur in a certain route. However, even in such cases this route may not be selected by the above-described conventional route selection methods, and consequently data may be concentrated in one particular route or a plurality of routes. That is, there is the possibility that an empty route may not be selected, because the transmission distance is long or the number of hops is large.
Further, in conventional route selection methods in which data is concentrated in a given route, if a currently utilized route is cut off, route switching is necessary for a large amount of data at once, and this may be the cause of a higher probability of congestion.
Even in cases in which the optical transmission device transmitting WDM signals and the optical receiving device receiving WDM signals comprise an operation system and standby system, the same route is adopted by the operation system and by the standby system, so that the above-described problem with route selection methods cannot be resolved. In this case, only defects within the operation system of the optical transmission device and optical receiving device can be accommodated.